The present invention relates to a target classification method for a radar system.
German Patent Applications DE 2,928,907.A1 (published Jun. 19, 1981) and DE 3,001,148.A1 (published Oct. 5, 1981), which are both related to U.S. Pat. No. 4,348,674, disclose methods for the classification of moving targets in which, in a Doppler radar system, Doppler spectra are formed from the echo signals and these spectra, in addition to forming the basis of a determination of the target velocity from the Doppler frequency of the primary target line in the Doppler spectrum, are examined for secondary lines originating from periodic changes of the reflection behavior of the target or parts of the target. For example, the blade repetition rate of the engine rotor of an aircraft jet engine or the length of a tank""s track-laying chain links can be determined from the spacing of the secondary lines, and this information can be utilized to classify the target. However, the blade repetition rate of an aircraft engine is only conditionally suitable as a classification feature since the rate of rotation of the rotor may vary over a broad range.
Target classification methods are of great significance for noncooperative identification Friend or Foe (IFF), for example for systems operating with exclusive utilization of the information furnished by the on-board radar system of an aircraft.
It is an object of the present invention to provide such noncooperative target classification methods.
The above object is achieved according to a first embodiment of the invention by a target classification method including comparing target information extracted from the echo signals of a Doppler radar signal with stored patterns, with a jet aircraft being classified by identification of its engine type, and including transforming the radar echo signals to a Doppler Spectrum signal by coherent processing, determining a value for the blade repetition rate of an engine rotor of the target from the Doppler spectrum, and using the blade repetition rate for identification; wherein the method further comprises: additionally analyzing those components of the echo signal modulation in the Doppler spectrum which are periodic with the rotation of the rotor and determining therefrom a value for the rate of rotation of the rotor; determining the number of blades of the engine rotor from the blade repetition rate and the rate of rotation; and utilizing the determined number of the blades of one engine rotor as a classification parameter during the step of comparing.
The above object is achieved according to a further embodiment of the invention by a target classification method utilizing the echo signal of a Doppler radar including comparing target information extracted from the echo signal of a Doppler radar with stored patterns, which method further comprises transforming the radar echo signals to a Doppler spectrum signal by coherent processing; and classifying a helicopter by: freeing the Doppler spectrum signal of the primary target echo (Fuselage echo); retransforming the remaining Doppler spectrum signal to a time domain signal; determining the blade repetition duration of the primary or tail rotor of the helicopter from the retransformed time domain signal; and utilizing the determined blade repetition determination for classification during the step of comparing.
The above object is achieved according to a still further embodiment of the invention by a target classification method utilizing the echo signal of a radar system, and including comparing target information extracted from the echo signal of the radar system with stored patterns, and further including: using a plurality of measured values of the radar echo signal to detect the type of modulation; obtaining at least one or a plurality of measured variables from each measured value; determining the statistical distribution of the measured variables in all measured values; and comparing the statistical distribution patterns with stored patterns.
While the blade repetition rate of the engine rotors of jet aircraft, which may vary over a broad range as a function of the momentary load on the engine, is evaluated in the prior art, the present invention is based on the idea of also determining the rate of rotation of the engine rotors. Although this rate also varies over a broad range, the quotient of the blade repetition rate and the rate of rotation is constant for one and the same engine type and equals the number of blades of the rotor. Determination of the target parameters of blade repetition rate and rate of rotation and their combined evaluation and formation of a quotient from the two parameters and comparing it with stored patterns thus makes it possible to unequivocally identify the engine type independently of the varying momentary operating state of the engine.
The invention utilizes the realization that the reflection behavior of an engine in successive, comparable rotor blade positions coincides substantially, though not completely, so that the echo signal receives a modulation which is periodic with a complete revolution of the engine rotor.
By suppressing the primary target echo in the Doppler spectrum as well as possibly existing clutter components, the signal components which are periodic with the revolution time of the engine rotor wheel become more distinct. Preferably, the blade repetition rate is determined from the Doppler spectrum in a known manner and spectral lines originating from the blade repetition modulation are eliminated so that the remaining spectrum is determined to a significant degree by the modulation caused by the rotation of the rotor wheel.